1. Field of the Invention
This invention relates to a phase type focusing screen and more particularly to a phase type focusing screen having a diffusing characteristic highly suited for forming an object's image in a photographic camera, a video camera or the like.
2. Description of the Related Art
Phase type focusing screens of varied kinds have hereto been proposed. They have a plurality of minute patterns of a given shape formed by protrudent or recessed parts provided on the surface of a transparent base plate. They are thus arranged to have a desired diffusing characteristic which imparts phase differences areawise to light fluxes passing therethrough.
For example, Japanese Laid-Open Patent Application, No. SHO 62-269127 has disclosed a phase type optical device which has minute patterns regularly and repeatedly arranged on the surface of a base plate 91 with each pattern formed by a rise 93 and a recess 94 as shown in FIGS. 10(A) and 10(B) of the accompanying drawings. However, the periodic array of the minute patterns tends to cause a chromatic blur in the image obtained by a photo-taking system as it brings about a strong influence of diffraction of light.
FIGS. 11(A) and 11(B) show another known focusing screen which has a stepped level difference equal to a distance between the base plate and a raised surface of a focusing screen that would produce a phase difference of .pi. radians between light traversing the raised region and light traversing the unraised region of the base plate. However, this focusing screen gives a desired diffusing characteristic only for one wavelength within the visible region. FIG. 11(B) is a sectional view of FIG. 11(A). Further, broken lines in FIG. 11(A) represent virtual lines. Each square region 101 defined by these vertical lines represents a unit region. The unit regions 101 are regularly arranged.
Japanese Patent Publication No. SHO 56-47539 has also disclosed a phase type optical system which is provided with a transparent optical element including a plurality of minute optically uneven bodies. In the case of this phase type optical system, a product of a difference between the refractive index of the uneven body (medium) and the refractive index of the outside thereof and the dimension of the uneven body is smaller than one wave length. A desired diffusing characteristic is obtained by arranging each of the uneven bodies to measure 10 .mu.m or less in the direction perpendicular to the incident direction of light. However, this phase type optical system is very difficult to manufacture as the uneven bodies must have an extremely thin thickness. In addition to this problem, the desired diffusing characteristic is obtainable only for one wave length. For other wave lengths, the diffusing characteristic varies.
Japanese Patent Publication No. SHO 60-53841 has disclosed a diffusing plate having a random phasic level array. This diffusing plate is arranged to allot, to each of regions obtained by dividing the surface of a base plate 111 into a checkered state consisting of M rows X N columns, one of a plurality of phase levels, which can be expressed as follows: EQU 0, .pi./K, 2.pi./K, 3.pi./K, . . .
(wherein K is at least 3)
In the case of this phase type optical device, a phase difference 0 to 2.pi. is equally divided into six equal portions or thereabout. Therefore, the degree of accuracy required for stepped difference control becomes severe.